class Solution:
  def _permute(self, s, idx, cur_res, result):
    if idx == len(s):
      result.append(cur_res)
    else:
      if s[idx].isalpha():
        self._permute(s, idx + 1, cur_res + s[idx].swapcase(), result)
      self._permute(s, idx + 1, cur_res + s[idx], result)

  def findLetterCaseStringPermutations(self, str):
    permutations = []
    self._permute(str, 0, '', permutations)
    return permutations